The present invention relates to the production of coatings of improved thermal resistance on articles of aluminum or aluminum alloys. It is particularly directed to the provision of coatings of high resistance to thermodynamic ablation on aluminum components, e.g., fins and windshields, of projectiles, especially hypervelocity kinetic energy projectiles. Military projectiles of this type, such as the 105mm XM735 and XM774 projectiles, have been recently developed to provide a particularly effective type of anti-tank weapon. Such projectiles normally travel at considerably higher than supersonic speeds.
Various methods have been employed to prevent aerodynamic thermal ablation on aluminum components, notably fins, of such hypervelocity projectiles. Prior to the present invention the most effective method to this end involved coating the aluminum fins with a thick, dense, hard anodic finish, commonly known as a "hard coating", which possesses excellent hardness and resistance to heat and abrasion and is obtained essentially by the anodic oxidation of aluminum using a refrigerated, acid electrolyte maintained below 10.degree. C. However, the hard coatings thus obtained showed evidence of breakdown when such projectiles were fired at extended ranges. When such a hard coating is broken down, the base aluminum erodes very rapidly from the combined heat and friction with the airstream. Once ignited, the aluminum burns synergistically, thereby destroying the basic aerodynamic shape of the component. Asymmetric drag is induced on badly ablated aluminum parts, which causes the projectile to become unstable and thus adversely affect accuracy.